The present invention relates generally to electrically conductive body straps for the control of electrostatic charges.
Persons who are around electrostatic sensitive articles need to be protected from acquiring or retaining an electrostatic charge. A person who acquires and retains a buildup of an electrostatic charge and who comes near to or in contact with an electrostatic sensitive article, e.g., an integrated circuit or electronic device, may deliver a sudden electrical discharge through the article severely damaging or destroying it. One mechanism to protect such electrostatic sensitive article is to electrically ground the body of the person who may come near to or in contact with the articles. Electrically conductive body straps, typically contacting the wrist, are utilized to electrically couple to the body of the person. The conductive body strap may then be electrically connected to ground in order to drain to electrical ground any electrostatic charge existing or generated on the body of the person.
There are several examples of electrically conductive body grounding straps in existance.
One of these straps is the Model 2064 wrist strap manufactured by Minnesota Mining and Manufacturing Company (3M), St. Paul, Minn. The Model 2064 wrist strap consists of a Velostat (Velostat is a trademark of the 3M Company) strip held on the wrist with a band of nylon. Velostat conductive material is a carbon loaded conductive polymer. The operation of the wrist strap relies on the conductive polymer to conduct electrostatic charge via the individual's wrist to a ground cord secured to the wrist strap with an electrically conductive snap connection. The Model 2064 wrist strap relies on a hook and loop fastener system (e.g., Scotchmate, a trademark of the 3M Company, fastener) to secure the wrist strap to the wrist of the individual wearer.
A wrist strap manufactured by Semtronics Corporation, Peachtree City, Ga. is constructed from similar functional components. The Semtronics carbon loaded wrist strap also uses a black conductive plastic secured to the wrist with a hook and loop closure system.
A wrist strap manufactured by Simco, Lansdale, Pa. also uses a similar system. The entire band of the wrist strap is made of a nylon hook and loop fastener system. The Simco wrist strap has a carbon loaded conductive material secured to the inner surface of the hook and loop fastener. A snap connection is provided for a ground cord. The Simco wrist strap again relies on the conductive polymer for conducting the electrostatic accumulation on the individual to the snap connection and to the grounding cord. The Simco wrist strap relies on the hook and loop fastener to close the wrist strap on itself and thereby secure the strap to the wrist of the wearer.
A wrist strap manufactured by Wescorp of Mountainview, Calif. consists of a carbon loaded conductive fabric with a hook and loop fastener. The Wescorp wrist strap relies on the conductive fabric for the conduction of electrostatic charge from the individual instead of the conductive polymer as in the previous straps but again relies on the hook and loop fastener for the closure system.
A strap manufactured by Walter G. Legge Company, New York, N.Y. carrying the name "WRISTSTAT" uses a black nylon band with a hook and loop fastener. A conductive polymer is attached to the band with a metallic plate at a relatively narrow location around the strap. The conductive polymer also has a snap connection to a ground cord. The Legge wrist strap relies on the metal plate and the conductive polymer for conductivity and relies on the nylon band with the hook and loop fastener for the closure system.
The straps heretofore described are all very similar in nature. Almost all rely on a carbon loaded conductive polymer and the remainder on a carbon loaded fabric. All of the wrist straps rely on a hook and loop fastener for a closure system.
Wescorp also has a strap consisting of a metallic bead chain to which an electrical ground cord is slidably attached. The strap relies on the metallic beads for conductivity. Since it is worn loosely around the wrist, it can be made large enough to slip over the hand onto the wrist and, thus, no detachable closure is required. This strap, however, does not ensure proper electrical connection since the strap is not intimately in contact with the body (wrist) of the individual wearer.
Controlled Static Company, Santa Fe, Calif. manufactures a wrist strap known in the trade as a Fred strap. The strap is a metallic expansion band having a snap connection for an electrical ground cord. The band is reminiscent of a metallic expansion watchband. The band relies on the conductivity of the metal for the drainage of the accumulated electostatic charges and will expand, slip on the wrist over the hand and then fit relatively snug. However, the wrist strap suffers the disadvantage of a relatively low expansion ratio. The strap must be large enough to slip over the hand and small enough to fit snugly on the wrist.
Further, the two previous metallic straps also suffer another significant disadvantage since the highly conductive metallic surface is available at the outside surface of the wrist strap, there is a danger of accidental contact with a high voltage source and the resultant "welding" of the strap to that source preventing disengagement of the wearer from the high voltage source. It is for this reason that some electricians do not wear metallic rings, bracelets and other jewelry.
U.S. Pat. No. 4,398,277, Christiansen et al, assigned to 3M, describes an electrically conductive elastomeric fabric and wrist strap. In Christiansen et al, a knitted fabric with elastomeric yarns and electrically conductive yarns is secured in a clasp with a snap for a ground wire connection. The knitted fabric is extensible to slip over the hand and yet fits snugly on the wrist. This strap operates extremely well; however, the cost of construction, in particular the silver thread utilized in the electrically conductive yarn, mitigates against this strap being utilized in a onetime use disposable application.
U.S. Pat. No. 4,577,251, Breidegam, discloses an electrically conductive elastomeric wrist strap. The fabric contained in the strap is a stretch weave with stretched longitudinal conductive threads. The fabric is adjustably secured into a clasp to form a closed loop in circling the wrist of the wearer. A snap is provided for an electrical ground wire connection. The Breidegam strap is somewhat extensible to slip over the hand, it is then pulled tight and secured to fit snugly around the wrist. Again, the cost of construction of this strap mitigates against its onetime use disposable application.
The closure system on the strap described in U.S. Pat. No. 3,857,397, Broeseonis is formed with the end of the strap having an opening. The strap is placed around the wrist with the one end brought back through the opening on the other end and snugged tight up against the wrist and secured in that position with a hook and loop fastener system. The opening in the end of the strap to bring the other end of the strap through makes it cumbersome to use on the wrist. The cost of construction mitigates against a onetime use disposable application.
A disadvantage of all the previous disclosed straps are that all of these straps are relatively expensive. Wrist straps must be inventoried and stored at the point of use by the potential wearer who comes in close contact with electrostatic sensitive articles. The wrist straps once used and worn must be maintained to ensure their proper functioning. Over a period of time, the electrical performance of any strap may deteriorate and, thus, the straps must be repeatedly tested to ensure for proper functionality. The cost of inventorying, storing and testing such wrist straps may limit the applications for which they can be used.